1. Field
This invention relates generally to cooling systems used to cool water in a nuclear reactor power generating facility and, more particularly, to a temporary cooling system which is designed to supplement existing cooling systems and accommodate emergency conditions experienced in such a facility.
2. Related Art
In nuclear power generating facilities, a reactor vessel housing nuclear fuel and water is positioned in what is commonly referred to as a refueling cavity or a reactor cavity. During power generation, a primary fluid coolant, normally water, is heated by the nuclear fuel, and the heat is used to generate steam for electric power generation. During shutdowns for refueling and other periods when the reactor is not operating, the decay heat from the fuel continues to heat the water in the reactor vessel. The water must be cooled to a desired level before the fuel may be removed from the vessel and transferred to a spent fuel pool of the facility by way of the reactor cavity. The residual decay heat generated by the reactor core is cooled during shutdown by a permanently installed residual heat removal system. The residual heat removal system provides heat exchange cooling for the decay heat coming from the fuel in the reactor core during shutdown. The heat removal capacity of this system is necessarily large. During normal shutdown, the residual heat removal system is operated for a number of days in order to remove the decay heat from the fuel to a point where the fuel may be removed from the core. This is due to the fact that the spent fuel pool, the eventual storage place for the fuel, has a permanently installed cooling system, which does not have sufficient cooling capacity to remove the high level of residual heat generated by the fuel immediately following plant shutdown.
Thus, in situations requiring removal of the fuel from the reactor core, the permanent cooling system configuration in present day nuclear plants requires that the residual heat removal system be operated for a period of days in order to cool the fuel to a point where it can be safely removed to the spent fuel pool, to permit reactor servicing, such as refueling or the decontamination of components such as the reactor recirculation system. U.S. Pat. No. 5,268,942 describes an auxiliary cooling system that can be permanently stored within the containment to augment the residual heat removal system and speed up that process. Without such an auxiliary system, the servicing crew must wait several days until the residual heat removal system adequately cools the reactor core before proceeding to remove the fuel. This cool down time increases the facility's overall shutdown period, thus increasing the cost of the shutdown operation, resulting in lost revenues, as well as the cost of replacement power purchased during the shutdown. On the other hand, the cost of such an auxiliary system or the cost of increasing the capacity of the spent fuel pool cooling system is expensive.
The Mar. 11, 2011 earthquake and tsunami in Japan lead to severe damage and station blackout at the Fukushima Dai'Ichi nuclear plant. Although the plant was successfully shut down after the earthquake, the ensuing tsunami rendered the plant unable to restore power to the cooling systems responsible for cooling the reactor and the spent fuel pool. This resulted in core meltdowns in three units, loss of water inventory, potential fuel failures in the spent fuel pools, and radioactive releases into the environment. Increased scrutiny is being focused on the world's nuclear power plants and the need to be able to respond to events that are beyond the nuclear plants' original design basis. This invention expands upon the capability of the system described in U.S. Pat. No. 5,268,942 in a way that enables an auxiliary cooling system to be rapidly mobilized and installed to provide a self-contained, self-sufficient means of removing decay heat from the spent fuel pool until normal plant equipment can be returned to service.
Thus, it is an object of this invention to provide a mobile cooling system that can be readily transported to a nuclear plant site and rapidly installed and activated to provide adequate cooling for the spent fuel pool.
It is a further object of this invention to provide such a cooling system that is self-contained, and is a self-sufficient means of removing decay heat from the spent fuel pool until normal plant equipment can be returned to service.
It is an additional object of this invention to provide such a cooling system at a reduced cost that can be spread over a number of power generating facilities at which it can be applied.
Further, it is an object of this invention to provide such a system that can be used to supply make-up water to a spent fuel pool in which the primary coolant has been depleted.